I think the rationale behind having a jury of "ordinary" people is they are usually not connected to the people who make the laws, which reduces the chance of making politically motivated verdicts. Civilian jurors may also be more likely to really consider the impact of sending someone to prison when making their verdict while professional jurors and judges may consider it more "routine" to send someone to prison. Yes, being forced into jury duty is inconvenient for people, but if it were a volunteer or full-time position, then it would inevitably become political.